Such a method is described in J. Electrochem. Soc., Solid-State Science and Technology 1983, pp. 665 to 659. Grooves must be formed in the manufacture of, for example, integrated circuits or for digital optical gratings. The etched depth must be monitored and measured. A device which is very suitable for this purpose is a laser interferometer shown in FIG. 3 of the above-mentioned paper.
In particular for the manufacture of the grooves of high-quality digital optical phase gratings (see J. Appl. Phys. 50 (6) 1979, pp. 3841 to 3848) the desired depth of the grooves must be etched with an accuracy of approximately .+-.0.25%.
Up to now an etch mask was photolithographically provided on a homogeneous SiO.sub.2 substrate layer, which mask does not cover the areas of the substrate layer where grooves are to be etched. Subsequently, an etching gas (for example CHF.sub.3) was used to etch the grooves, the total depth of the grooves, i.e., from the surface of the masking layer to the bottom surface of the grooves, being monitored by means of a laser interferometer. In this method the really essential value of the desired depth of the grooves formed in the substrate layer could not be determined directly but instead a value is provided in which the residual thickness of the masking layer is included. If the etch rate of the substrate material and the etch rate of the masking material, which is generally substantially lower than that of the substrate material, are known, the etch depth in the substrate layer can be calculated from the total depth of the groove being formed. Unfortunately, however, the measuring inaccuracy is higher than 5% because the materials of the mask and the substrate may have a different ratio of the etch rates which is governed by, inter alia process parameters.